Water purification is of considerable interest in many industries. For example, in the pharmaceutical industry, so-called “ultra-pure” water (i.e., water having a resistivity of 18.2 megaohms/cm) is used in many of the reactions either under study for researching new drugs or involved in drug manufacture. An inordinately high concentration of ions and other impurities in such water can affect negatively such reactions, introducing sources of error that can result in misdiagnosis or otherwise flawed data.
Electrodeionization is a process for removing ions from liquids by sorption of these ions into a solid material capable of exchanging these ions either for hydrogen ions (for cations) or hydroxide ions (for anions) and simultaneous or later removal of the sorbed ions by the application of an electric field.
The electrodeionization process is often conducted in an apparatus consisting of alternating diluting compartments and concentrating compartments separated by anion and cation permeable membranes. The diluting compartments are filled with a porous ion exchange materials through which the water to be deionized flows. The ion exchange materials are commonly mixtures of cation exchange resins and anion exchange resins. Alternating layers of these resins have been suggested. Ion exchange materials consisting of woven and non-woven fibers have also be disclosed. The compartments adjoining the diluting compartment into which the ions are moved by the applied electric field, called concentrating compartments, may be filled with ion exchanging materials or with inert liquid permeable materials. An assembly of one or more pairs of diluting or concentrating compartments, often referred to as a “cell pair”, is bounded on either side by an anode and a cathode, thereby enabling the application of an electric field perpendicular to the general direction of liquid flow.
Although past electrodeionization devices are effective and provide good results, need continues for further improvement thereto, and in particular, for increasing the capacity and throughput of such devices, along predictable linearly-scaled increments, without compromising good “ultra-pure” deionization.